Prediction on temperature dependent elastic constants of “soft” metal Al by AIMD and QHA

First-principles methods based on density functional theory(DFT) are nowadays routinely applied to calculate the elastic constants of materials at temperature of 0 K. Nevertheless, the first-principles calculations of elastic constants at finite temperature are not straightforward. In the present work, the feasibility of the ab initio molecular dynamic(AIMD) method in calculations of the temperature dependent elastic constants of relatively "soft" metals, taking face centered cubic(FCC) aluminum(Al) as example, is explored. The AIMD calculations are performed with carefully selected strain tensors and strain magnitude. In parallel with the AIMD calculations, first-principles calculations with the quasiharmonic approximation(QHA) are performed as well. We show that all three independent elastic constant components(C_(11), C_(12) and C_(44)) of Al from both the AIMD and QHA calculations decrease with increasing temperature T, in good agreement with those from experimental measurements. Our work allows us to quantify the individual contributions of the volume expansion, lattice vibration(excluding those contributed to the volume expansion), and electronic temperature effects to the temperature induced variation of the elastic constants. For Al with stable FCC crystal structure, the volume expansion effect contributes the major part(about 75%～80%) in the temperature induced variation of the elastic constants.The contribution of the lattice vibration is minor(about 20%～25%) while the electronic temperature effect is negligible. Although the elastic constants soften with increasing temperature, FCC Al satisfies the Born elastic stability criteria with temperature up to the experimental melting point.     

Annealing Effect on Transport and Magnetic Properties of La_(0.67)Sr_(0.33)MnO_3 Thin Films Grown on Glass Substrates by RF Magnetron Sputtering

The manganite La0.67Sr0.33MnO3(LSMO) thin films were grown on glass substrates in a mixed argon and oxygen atmosphere by using RF magnetron sputtering.The structural characteristics,transport behaviors and magnetic properties of LSMO films were studied by annealing the films in air at 550 and 620 ℃.The out-of-plane lattice parameter a LSMO contracted after annealing and was close to that of bulk LSMO abulk,indicating that the internal strain was fully relaxed.Nanocrystalline grains were observed in the annealed films.Enhanced saturation magnetization and metal-to-insulator transition temperature(TMI=268 K) were also obtained.Curie temperatures(Tc) of the as-grown films was 340 K with the same as that of annealed at 550 ℃,but dropped to 315 K when the annealing temperature increased to 620 ℃,which can be attributed to the oxygen release during annealing in atmosphere.     

Magnetic Ordering and Exchange Interaction of Laves Compounds Sm0.9Pr0.1（Fe1—xCox）2

Structure,magnetic properties and magnetostriction of Sm0.9Pr0.1(Fe1-xCox)2 compounds have been investi-gated by means of X-ray diffraction,a.c. initial susceptibility, extracting sample magnetometer,Mossbauer spec-troscopy and standard strain gauge techniques.The lattice parameter a of the MgCu2-type Laves compounds Sm0.9Pr0.1(Fe1-xCox)2 decreases nonlinearly with increasing Co concentration,deviating from the Vegardˊs law.Curie temperature Tc increases initially from 668 K for x=0 to 694 K for x=0.2 and then decreases to 200 K for x=1.0.The saturation magnetization Ms at temperatures 1.5K, 77K and 300K have the same variation tendency as the composition dependence of Curie temperature,in consistence with rigid-band model.The easy magnetization direction(EMD) od Sm0.9Pr0.1(Fe1-xCox)2 lies along [111] direction in the range x≤0.6,and changes to [110] for x=0.8 ,while Sm0.9Pr0.1(Fe1-xCox)2 stays in the paramagnetic state at room temperature.The composition dependence of the average hyperfine field,Hhf,demonstrates a similar variation tendency as that of the saturation magnetiza-tion Ms and Curie temperature Tc .The spontaneous magnetostricton λ111 increases with increasing Co content.The saturation magnetostriction λs decreases monotonically with increasing x,which is caused by the increase of magnetostriction constant λ100 with opposite sign to that of λ111.A two-sublattice model has been proposed to understand the intermediate region between the [111]and [110] spin configurations ,which can also be used to explain the temperature dependence of magnetization.     

Raman Scattering and t-Phase Lattice Vibration of 3% (mole fraction) Y2O3-ZrO2

The Raman spectra of 3% (mole fraction) Y2O3-ZrO2 (3Y) are obtained at different temperature from 77 K to 853 K. The results show that 260 cm-1 Eg and 460 cm-1 Eg modes on the spectra shift toward lower wave number with the increase of temperature; meanwhile, the continuity of the Half Width at Half Maximum (HWHM) and wave number (frequency) of Raman bands are broken and t-phase only partially transforms to m-phase at 523 K. Based on the experimental results, this paper discusses the t-phase lattice vibration of 3Y and presents the images of vibration displacement of six Raman-active modes for t-phase of ZrO2. The analysis indicates that there are diversities existing in the displacement of the atomic vibration of Raman-active modes because of their different symmetries at various temperature, which leads to the different change tendencies of Raman bands. Furthermore, the abrupt changes of some Raman-active modes indicating the atomic displacement are assumed to be the condensations of phonon modes in t     

Structure and Thermal Parameters of Ni20Pd80 Alloy

The structure and thermal parameters of Ni20Pd80 alloy were studied by X-ray diffraction(XRD). The diffraction experiments performed in the temperature range of 308–1100 K revealed that the alloy formed a face centered cubic (fcc) A1-type structure. The temperature dependence of the lattice parameters was investigated by using the Bragg line displacement method showing that the lattice parameter increases with the increase of temperature. The mean linear thermal expansion (MLTE(%)), coeffcient of thermal ex...     

Investigation on Superplasticity in SiCp／2024 Cold Rolling Sheet after Heat Treatment

High strain rate superplastic deformation behavior of powder metallurgy (PM) processed 17 vol. pct SiCp/2024 Al composite sheet after heat treatment was investigated over a range of temperature from 753 to 833 K. At 813 K,a maximum elongation of 259% was discovered at a strain rate of 10^-1 s^-1. The activation energy was closed to that for lattice diffusion of Al and increased at temperature upon incipient melting temperature. The mechanism of superplastic deformation for present composites was attributed to lattice diffusion controlled grain boundary sliding.     

Improving the Thermal Stability of Cu_3N Films by Addition of Mn

Mn-doped Cu_3N films were deposited by cylindrical magnetron sputtering equipment on the common glass at room temperature.The incorporation of Mn can change the preferred growth orientation from Cu-rich plane(111) to N-rich plane(100) due to the improvement of nitridation of Cu.The shrinkage of the lattice and the X-ray photoelectron spectroscopy results reveal that Mn should replace Cu atoms in the lattice or be segregated in the grain boundaries.The thickness of Mn-doped film is smaller than that of the pure one due to the less physisorption of N species among the columnar grains.The mean grain size and the energy gap become larger with increasing Mn concentration to 2.2 at.%and then decrease when the concentration of Mn is higher than 2.2 at.%.Notably,weak doping of 1.5 at.%Mn successfully promotes the decomposed temperature by~50℃.According to the results of XRD and SEM for Mndoped films annealed in vacuum,a possible decomposed mechanism with increasing the annealing temperature is proposed.     

Glass-to-Rubber Transition of Polymer Thin Films and Their Surface Dynamical Properties

Glass-to-rubber transition temperature, Tg' of polystyrene(PS) (Mw=500K, Mw/Mn=1.03)thin films (thickness, d= 100 to 2000 A) deposited on Si with native oxide was determined by variable angle spectroscopic ellipsometry(VASE. We observed that the Tg of the polymer films decreased monotonically as the film thickness was decreased. It had previously been proposed that this was due to a highly mobile surface rubbery layer that existed even well below Tg' We used atomic force microscopic(AFM)adhesion measurement as a direct probe to investigate the surface dynamical properties of the PS samples and a thin film ofpoly(tert-butyl acrylate) (PtBA) (Mw= 148K, Mw/Mn=17, and Tg bullk=50℃). By comparing the AFM results and those obtained from shear modulus measurements of a bulk sample, we found no enhancement in the molecular relaxation at the free surface of these samples.     

Hot deformation behavior and processing map development of AZ110 alloy with and without addition of La-rich Mish Metal

In order to compare the workability of AZ110 alloy with and without addition of La-rich Mish Metal(MM), hot compression tests were performed on a Gleeble-3500 D thermo-mechanical simulator at the deformation temperature range of 473-623 K and strain rate range of 0.001-1 s-1. The flow stress, constitutive relation, DRX kinetic model, processing map and microstructure characterization of the alloys were investigated. The results show that the flow stress is very sensitive to deformation temperature and strain rate, and the peak stress of AZ110 LC(LC = La-rich MM) alloy is higher than that of AZ110 alloy.The hot deformation behavior of the alloys can be accurately predicted by the constitutive relations. The derived constitutive equations show that the calculated activation energy Q and stress exponent n for AZ110 alloy are higher than the calculated values of AZ110 LC alloy. The analysis of DRX kinetic models show that the development of DRX in AZ110 LC alloy is earlier than AZ110 alloy at the same deformation condition. The processing maps show that the workability of AZ110 LC alloy is significantly more excellent than AZ110 alloy and the microstructures are in good agreement with the calculated results.The AZ110 LC alloys can obtain complete DRX microstructure at high strain rate due to its higher stored energy and weak basal texture.     

Effect of Cerium Doping on Microstructure and Dielectric Properties of BaTiO3 Ceramics

A solid state reaction method was used to synthesize barium titanate (BT) and barium cerium titanate (BCT) ceramics at sintering temperature of 1473 K for 4 h. The effect of cerium (Ce) on the structure, microstructure and dielectric properties of BCT was investigated. The scanning electron microscopy (SEM) investigations revealed that the grain size increases with increasing Ce content. The X-ray diffraction (XRD) patterns showed mostly the BT phase, where the lattice parameter decreased with the addition of Ce. The temperature dependence of dielectric constant showed decrease in the phase transition temperature with higher Ce content. The dielectric constant decreased slightly with increasing frequency. The direct current (dc) density-voltage characteristics of the ceramics showed ohmic behavior for both the BT and BCT. As the temperature increased, the dc resistivity of the ceramics decreased. The activation energy increased with increasing Ce content.     

Martensitic Transformation in Magnetically Controlled Shape Memory Alloys Co50Ni20Ga30

The martensitic transformation for Co50Ni20Ga30 ribbon synthesized by the melt-spinning technique was studied by means of X-ray diffraction and ac magnetic susceptibility. The Co50Ni20Ga30 ribbon, having bcc phase with calculated lattice parameters of a=0.57431 nm at 313 K. It exhibits a structure transition from parent phase to martensite during cooling. The martensitic phase in Co50Ni20Ga30 ribbon is tetragonal structure with lattice parameters of a=b=0.5422 nm and c=0.6401 nm. (c/a>1). According to the changing of diffraction intensity for martensite and the change of ac magnetic susceptibility, the process of the martensitic transformation can be divided into three parts during cooling from 283 K to 213 K. When the temperature decreasing sequentially from 193 K to 110 K, the structure of the martensite has a change in which the a-axis decreases and c-axis increases. The morphologies of selfaccommodation were observeds. The parallelogram morphology, the diamond morphology and the fork morphology were     

Glass-to-Rubber Transition of Polymer Thin Films and Their Surface Dynamical Properties

Glass-to-rubber transition temperature, Tg' of polystyrene(PS) (Mw=500K, Mw/Mn=1.03)thin films (thickness, d= 100 to 2000 A) deposited on Si with native oxide was determined by variable angle spectroscopic ellipsometry(VASE. We observed that the Tg of the polymer films decreased monotonically as the film thickness was decreased. It had previously been proposed that this was due to a highly mobile surface rubbery layer that existed even well below Tg' We used atomic force microscopic(AFM)adhesion measurement as a direct probe to investigate the surface dynamical properties of the PS samples and a thin film ofpoly(tert-butyl acrylate) (PtBA) (Mw= 148K, Mw/Mn=17, and Tg bullk=50℃). By comparing the AFM results and those obtained from shear modulus measurements of a bulk sample, we found no enhancement in the molecular relaxation at the free surface of these samples.     

Mechanically Driven Alloying and Structural Evolution of Nanocrystalline Fe_(60)Cu_(40) Powder

Highly supersaturated nanocrystalline fcc Fe60Cu40 alloy has been prepared by mechanical alloying of elemental powders. The phase transformation is monitored by X-ray diffraction (XRD),Mossbauer spectroscopy and extended X-ray absorption fine structure (EXAFS). The powder obtained after milling is of single fcc structure with grain size of nanometer order. The Mossbauer spectra of the milled powder can be fitted by two subspectra whose hyperfine magnetic fields are 16 MA/m and 20 MA/m while that of pure Fe disappeared. EXAFS results show that the radial structure function (RSF) of Fe K-edge changed drastically and finally became similar to that of reference Cu K-edge, while that of Cu K-edge nearly keeps unchanged in the process of milling. These imply that bcc Fe really transforms to fcc structure and alloying between Fe and Cu occurs truly on an atomic scale. EXAFS results indicate that iron atoms tend to segregate at the boundaries and Cu atoms are rich in the fcc lattice. Annealing experiments show that the Fe atoms at the interfaces are easy to cluster to α-Fe at a lower temperature, whereas the iron atoms in the lattice will form γ-Fe first at temperature above 350℃, and then transform to bcc Fe     

Fractal Nature of Crack Growth under Different Temperature

We have studied numerically a simple crack growth model in a two-dimensional triangular lattice of bonds and nodes which incorporates the sudece of a growing crack bond-breaking probability Pi~ exp{(-Vi+Ei)φ(T)}, where Ei is elastic eneny stored in the i-th bond. Different energy temperature factors φ(T) are employed during crack formation and propagation process with a uniform dilation strain case and a shear case and with periodic boundary condition in the horizontal direction. Our results show that the patterns of the cracks generated are fractal structure and the effective fractal dimensionalities decrease with the increase of the temperature factor φ(T)(the absolute temperature T decreasing). In the paper we also discuss the relation between the effective fractal dimension Deff (the radius Rg Of gyration) and the fractal dimensions D (the radius R of circular), and also give their modification values Ω about two kinds of methods in the lattice model     

Oxygen Relief/Absorption and Superconductivity of Bi-Pb-Sr-Ca-Cu-O Compounds

The oxygen relief/absorption,superconduc—tivity and structure of(Bi_(1-x)Pb_x)_2Sr_2Ca_2—Cu_3O_y compounds have been studied by TGA,electrical resistance measurement and X-raydiffraction.The amount of oxygen which can be re-leased and superconductivity of the compounds arecorrelative to Pb content.The release of oxygen in-creases when x less than 0.2 and decreases when xexceeds 0.2 with increasing x.The appropriate con-tent of Pb for superconductivity is in the range of0.1相似文献   

Magnetic and Electrical Behavior of MnTe_(1-x)Sb_x Alloys

The structure,specific heat,magnetic and electrical properties of MnTe1-xSbx(x=0,0.1,0.15,0.2 and 0.25) alloys have been investigated.The MnTe1-xSbx alloys crystallize in a hexagonal NiAs-type structure,and the impurity of MnSb phase appears when x≥0.15.The MnTe0.9Sb0.1 compound exhibits ferrimagnetic behavior with hysteresis loops even at 350 K,showing that the magnetic properties of MnTe compound are very sensitive to little compositional change.The ferromagnetism in the MnTe1-xSbx alloys with higher Sb contents may be attributed to the impurity of MnSb phase.Energy dispersive X-ray spectroscopy analysis on the MnTe0.9Sb0.1 compound indicates that Sb is very difficult to dope into the lattice of MnTe.So the anomaly of resistivity at 300 K of MnTe0.9Sb0.1 and the peak of specific heat around 304 K of all the alloys are thought to be related with the antiferromagnetic interactions of MnTe-based lattice.     

Electrochemically stable lithium-ion and electron insulators(LEIs)for solid-state batteries

Rechargeable solid-state Li metal batteries demand ordered flows of Li-ions and electrons in and out of solid structures,with repeated waxing and waning of Ubcc phase near contact interfaces which gives rise to various electro-chemo-mechanical challenges.There have been approaches that adopt three-dimensional(3D)nanoporous architectures consisting of mixed ion-electron conductors(MIECs)to combat these challenges.However,there has remained an issue of LiBcc nucleation at the interfaces between different solid components(e.g.,solid electrolyte/MlEC interface),which could undermine the interfacial bonding,thereby leading to the evolution of mechanical instability and the loss of ionic/electronic percolation.In this regard,the present work shows that the Li-ion and electron insulators(LEIs)that are thermodynamically stable against LiBcc could combat such challenges by blocking transportation of charge carriers on the interfaces,analogous to dielectric layers in transistors.We searched the ab initio database and have identified 48 crystalline compounds to be LEI candidates(46 experimentally reported compounds and 2 hypothetical compounds predicted to be stable)with a band gap greater than 3 eV and vanishing Li solubility.Among these compounds,those with good adhesion to solid electrolyte and mixed ion-electron conductor of interest,but are lithiophobic,are expected to be the most useful.We also extended the search to Na or K metal compatible alkali-ion and electron insulators,and identified some crystalline compounds with a property to resist corresponding alkali-ions and electrons.     

Morphology-Controlled Synthesis of CeO_2 Microstructures and Their Room Temperature Ferromagnetism

Gear-shape CeO_2 microstructures have been synthesized via a facile hydrothermal method with Ce(NO_3)_3·6H_2O as the cerium source, NH_4HCO_3 as both the precipitator and the carbon source, and cetyltrimethyl ammonium bromide(CTAB) as the surfactant. X-ray diffraction(XRD) inferred that the synthesized CeO_2 microstructures exhibited a fluorite structure. The band gap(Eg) of CeO_2 samples is larger than that of bulk. X-ray photoelectron spectroscopy(XPS) showed that there are plenty of Ce~(3+) ions and oxygen vacancies at the surface of CeO_2 samples. All the synthesized CeO_2 samples exhibited the room temperature ferromagnetism, and the saturation magnetization increases with the increases of lattice parameter and Eg. The room temperature ferromagnetism mechanism of gear-shape CeO_2 is mainly attributed to the influence Ce~(3+) ions.     

Conduction Properties and Scattering Mechanisms in F-doped Textured Transparent Conducting SnO_2 Films Deposited by APCVD

Transparent conducting F-doped texture SnO2 films with resistivity as low as 5× 10-4 Ω ·cm,with carrier concentrations between 3.5 × 1020 and 7× 1020 cm-3 and Hall mobilities from 15.7 to 20.1 cm2/(V/s) have been prepared by atmosphere pressure chemical vapour deposition (APCVD). These polycrystalline films possess a variable preferred orientation, the polycrystallite sizes and orientations vary with substrate temperature. The substrate temperature and fluorine flow rate dependence of conductivity, Hall mobility and carrier conentration fOr the resultingfilms have been obtained. The temperature dependence of the mobiity and carrier concentrationhave been measured over a temperature range 16～400 K. A systematically theoretical analysis on scattering mechanisms for the highly conductive SnO2 films has been given. Both theoretical analysis and experimental results indicate that for these degenerate, polycrystalline SnO2 :F films in the low temperature range (below 100 K), ionized impurity scattering is main scattering mechanism. However, when the temperature is higher than 100 K, the lattice vibration scattering becomes dominant. The grain boundary scattering makes a small contribution to limit the mobility of the films.     

Magnetic and Transport Properties of Mn_(0.98)Cr_(0.02)Te Epitaxial Films Grown on Al_2O_3 Substrates

The epitaxial Mn_(0.98)Cr_(0.02)Te films on single crystal Al_2O_3(0001) substrates were prepared by pulsed laser deposition.The X-ray diffraction and scanning electron microscopy results showed that the good continuous epitaxial film was obtained with substrate temperature of 500 ℃.When the substrate temperature reached700 ℃,the film was island growth and the manganese oxides phase appeared.The temperature dependence of both the magnetization and electrical resistance showed a sharp rise at around 60 K due to the magnetoelastic coupling.The temperature dependence of the electrical resistance of Mn_(0.98)Cr_(0.02)Te provided evidence for a transition from the metallic to semiconducting state at 305 K due to the spin disorder scattering with a large contribution from the influence of magnon drag.